INTRODUCING A FLUID INTO A CAVITY

Abstract: A centrifuge (18) is used to introduce a fluid, particularly a viscous fluid such as an encapsulating resin, into a cavity in a container (10), particularly a small cavity such as the inside of an electronic component housing. Measured quantities of the fluid are placed in dispensing vessels (28) in the centrifuge opposite the containers to be filled. The dispensing vessels have openings (36) through which the fluid can flow when centrifugal force is applied. On the application of this force, all the fluid flows out of the vessels into the containers.

Description: Introducing a fluid into a cavity This invention relates to a method of introducing a fluid into a cavity in a container, and also to an apparatus by which a fluid can be introduced into a cavity. The invention is envisaged for filling, casting, embedding, encapsulation, impregnation and potting applications. In some of such applications, typically a relatively high viscosity liquid is to be introduced into a small cavity, and a particular use for which the invention is envisaged is the introduction of encapsulating resins into housings for electronic components. The invention is not however limited to these applications and may also find use where it is necessary to accurately dispense small quantities of liquid, or flowable powders, into small cavities. It is known to make use of centrifugal force to assist the filling of mould cavities with fluid materials, for example molten metals or plastics. Such processes are known as "spin casting" or "centrifugal casting". It is also known from British Patent Specification 2 241 465 to use centrifugal force to assist the filling of components with encapsulating resin. However in this specification an overfilling technique is described, and the top face of the components is closed during filling. This has the disadvantage that it is necessary to snap off an injection nozzle from the filled and cured component. In prior art processes, a certain quantity of resin or other liquid remains in and hardens in a feed channel and is waste which is discarded. SUBSTITUTESHEET(RULE28) According to the present invention there is provided a method of introducing a fluid into a cavity in a container, the method comprising the steps of a placing into a dispensing vessel the quantity of fluid to be introduced into the cavity b placing the container and the dispensing vessel into a centrifuge so that the container cavity, when the centrifuge is operating, lies radially outside a dispensing opening of the dispensing vessel, c evacuating the centrifuge so that air is evacuated from the container before the resin is introduced, and d operating the centrifuge so that all the liquid flows out of the dispensing opening and into the cavity through centrifugal force. Although this invention is applicable to both liquids and fluid solids, for ease of description reference will hereafter be made to liquids. However the reader is to understand that fluids can be substituted for liquids, where appropriate. By charging the dispensing vessel with the exact quantity of liquid to be introduced into the cavity, the advantage is achieved that no liquid is wasted. This is of particular benefit when the liquid cures, because once curing has taken place, recycling of the liquid is no longer possible. Furthermore by suitably designing the dispensing vessel and its opening, it is possible to ensure that after operation the dispensing vessel needs no cleaning before being refilled with the next charge of liquid . In the case of the encapsulation of electronic components by a settable resin, it is important to ensure that no air is trapped in the resin and/or in the cavity. Preferably therefore the resin to be introduced into the cavity is degassed before being placed in the dispensing vessel. Air occupying the interior of the container may be displaced when the liquid is introduced under centrifugal force, but to ensure that there is no entrapment of air, the centrifuge is operated under vacuum so that air is evacuated from the container before the resin is introduced. It is also preferred for the dispensing opening of the dispensing vessel to be out of contact with the cavity being filled, at least during the time when the liquid flows out of the dispensing opening and into the cavity through centrifugal force. According to a second aspect of the invention there is provided apparatus for introducing a fluid into a cavity in a container, the apparatus comprising a dispensing vessel having a dispensing opening, the dimension of the dispensing opening being such that liquid will not flow therethrough under normal gravitational conditions but will flow therethrough when centrifugal force is applied, a vacuum centrifuge, and means for locating the dispensing vessel and the container to be filled in the centrifuge so that when the centrifuge is operating the cavity is evacuated and is located radially outward of the dispensing opening of the dispensing vessel and centrifugal force will result in the fluid being drawn out of the dispensing opening and flowing into the cavity. Conveniently the dispensing vessel comprises a cup with an open top and a narrow bore in its base to form the dispensing opening. A measured quantity of liquid can be placed in the cup through the open top, and will be prevented from flowing out through the opening under normal gravitational conditions by surface tension or suitable rheology. The centrifuge preferably includes a plurality of dispensing vessels and a plurality of container support locations so that a plurality of cavities can be filled in one centrifuge operation. The containers are preferably housings for electronic components and the liquid is preferably a conventional encapsulating resin as known for encapsulating electronic components. In a preferred embodiment the dispensing vessels and the container support locations are mounted together in swing- out buckets so that the open tops of the dispensing vessels are uppermost until the centrifuge starts to operate, so that there is no danger of the liquid running out of the open top of the dispensing cups before the centrifuge starts to operate. However when the liquid is particularly viscous or thixotropic it may be possible to use dispensing vessels which are radially directed at all times, rather than only during centrifuge operation. Preferably the step of measuring the quantity of liquid to be introduced into the dispensing vessel takes place under atmospheric conditions and the centrifuging operation takes place under vacuum conditions. The invention will now be further described, by way of SUBSTITUTE SHEET(RULE2β) example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an electronic component to be encapsulated; Figure 2 is a schematic view of apparatus in accordance with the invention,- Figure 3 is a detailed perspective view of part of the apparatus of Figure 2; Figures 4a, 4b and 4c are sections through the apparatus of Figure 3, showing three different stages in the operation of the apparatus; Figures 5a, 5b, 5c, 5d and 5e show five sequential stages in the operation of an alternative form of apparatus in accordance with the invention; Figures 6a and 6b show one method of measuring the quantity of liquid placed in the dispensing vessel; and Figures 7a, 7b and 7c show three sequential stages in the operation of a third embodiment of apparatus in accordance with the invention. Figure 1 shows an electronic component 10, before encapsulation. The component comprises an open topped housing 12 in which a toroidal, wound magnetic core 14 is placed. The windings around the core 14 are connected to component terminals 16. The component 10 as shown in Figure 1 is operatively complete, but it is normal to fill the space within the casing 12 with an encapsulating resin SUBSTITUTE SHEET(RULE2 to encapsulate the core 14 to protect the component and to fill all internal interstices with insulation. The invention is in no way restricted by the nature of the contents of the housing 12. The principle of encapsulation of such components is well known. However it is often difficult to perform satisfactory encapsulation, particularly with very small components. The resins used for encapsulation (typically filled epoxy resins although the man skilled in the art will be aware of the wide variety of resins which can be used for this purpose) are viscous and do not flow easily into small cavities. It is also necessary to ensure that the terminals 16 are kept clear of resin. Figure 2 shows a centrifuge generally designated 18 with a chamber 20 in which a vacuum can be maintained. The centrifuge has a rotor 22 and at each end of this rotor a frame 24 is mounted. When the rotor 22 rotates, the frames 24 are subjected to centrifugal force so that components mounted in the frame can be filled with resin, as will be described in the following. Figure 3 shows one of these cages 24 on a larger scale. The cage has an inner plate 26 in which a plurality of dispensing vessels 28 are formed. The cage also has an outer plate 30 with a mounting track 32 on which a plurality of components 10 are mounted. The components 10 are accurately positioned on a radial line of the centrifuge rotor passing through the centre of each of the dispensing vessels 28. In operation, resin 38 mixed with catalyst is charged into all the dispensing vessels 28 (see Figure 4a) . Each SUBSTITUTESHEET(RULE2β) - 1 - dispensing vessel has a cup-like chamber 34 and a narrow neck 36 which forms a dispensing opening. The rheology of the resin 38 in the cup 34 is designed to be sufficient to ensure that the resin does not drip out of the cup when the cup is oriented as shown in Figures 4. When the centrifuge operates and the cage 24 is spun, the resin is forced out through the opening 36 under centrifugal force and enters the component 10 as shown in Figure 4b. When all the resin has passed out of the cup 34 into the component, the component will be completely filled as shown in Figure 4c and the quantity of resin initially charged into the cup 34 will be the same as that required to fill the component 10 to the desired level. An apparatus as described is satisfactory for use where the resin has a high viscosity or thixotropy. However if the rheology of the resin is such that there might be a tendency for the resin to run out of the cup or component when the centrifuge has insufficient rotational speed, the cup and the associated component may be mounted together in a "swing-out bucket" arrangement as shown in Figures 5. Figure 5a shows two cups 34a and 34b located vertically above two components 10a and 10b. In this position a force equal to gravity acts on the mass of the resin, and this force is insufficient to cause the resin to flow through the dispensing opening 36. Alternatively, if the rheology of the resin and the shape of the cup and component are such that the resin will be retained in the component by surface tension, but might run out of the cup, then the cup can be fitted with a cap to retain the resin until centrifugal force is applied. If the centrifuging step is carried out under atmospheric conditions, the cap would need to be vented. However once the centrifuge rotor accelerates, the angle of the swing-out buckets progressively changes the orientation of the cups 34a, 34b and of the components 10a and 10b to the position shown in Figure 5b. Once the centrifuge reaches a threshold speed when a centrifugal force typically of lOOg is generated, the resin will flow through the openings 36 into the components as shown in Figure 5c. Once all the resin has flowed into the components as shown in Figure 5d, the speed of the rotor is gradually reduced to zero and the angle of the swing- out buckets is progressively reduced in a controlled fashion to the rest position shown in Figure 5e, to ensure that the resin does not run out of the component as the centrifuge slows down. It is an important feature of the invention that the quantity of resin charged into each cup 34 is that quantity required to precisely fill the component 10 to a desired level. If the metering of resin into the cup is carried out at atmospheric pressure, then considerable metering accuracy is possible. Alternatively however the exact quantity of resin can be introduced by dimensioning each cup 34 so that it will hold exactly the correct amount of resin, and then using a doctor blade 40 to wipe across the top of each cup to produce a uniform level of filling, as shown in Figures 6a and 6b. In another embodiment of apparatus in accordance with the invention, the centrifuge has four relatively rotatable rings. A first ring 42 carries the components 10. A second ring 44 is divided to define chambers 46 (Figure 7a) of specified dimensions to accommodate a precise volume of resin. The ring 44 can be moved relative to a third ring 48 between two positions to either admit resin from a central reservoir 50 to the chambers 46, or to close the chambers so that no resin can be admitted. The apparatus also includes a fourth ring 52 in which dispensing openings 54 are formed opposite the location of components 10. During the initial operation of the centrifuge, as shown in Figure 7a, the ring 44 is moved so that resin 54 can flow into the chambers 46. In a second stage shown in Figure 7b the ring 44 is moved so that the chambers 46 are closed and a defined volume of resin is trapped therein. In a third stage the ring 44 is moved until the chambers 46 line up with the dispensing openings 54, and in this position the centrifugal force arising from centrifuge rotation results in the resin being forced out of the chambers into the components 10, in a manner similar to that already described. The invention thus allows accurate and rapid filling of electronic components with a settable resin. As already pointed out however the invention is not limited to this application. It is a particular advantage of the present invention that the cycle time for the filling of each component is significantly less than that required by prior art methods.

Claims: Claims 1. A method of introducing a fluid into a cavity in a container, the method comprising the steps of a placing into a dispensing vessel the quantity of fluid to be introduced into the cavity b placing the container and the dispensing vessel into a centrifuge so that the container cavity, when the centrifuge is operating, lies radially outside a dispensing opening of the dispensing vessel, c evacuating the centrifuge so that air is evacuated from the container before the resin is introduced, and d operating the centrifuge so that all the liquid flows out of the dispensing opening and into the cavity through centrifugal force. 2. A method as claimed in Claim 1, wherein the fluid is a resin, and the resin to be introduced into the cavity is degassed before being placed in the dispensing vessel. 3. A method as claimed in any preceding claim, wherein the step of measuring the quantity of liquid to be introduced into the dispensing vessel takes place under atmospheric conditions and the centrifuging operation takes place under vacuum conditions. 4. A method as claimed in any preceding claim, wherein the walls of the dispensing vessel which form the dispensing opening are out of contact with the cavity being filled during the time when the liquid flows out of - li ¬ the dispensing opening and into the cavity through centrifugal force. 5. A method as claimed in any preceding claim, wherein the containers are housings for electronic components and the liquid is a conventional encapsulating resin as known for encapsulating electronic components. 6. Apparatus for introducing a fluid into a cavity in a container, the apparatus comprising a dispensing vessel having a dispensing opening, the dimension of the dispensing opening being such that liquid will not flow therethrough under normal gravitational conditions but will flow therethrough when centrifugal force is applied, a vacuum centrifuge, and means for locating the dispensing vessel and the container to be filled in the centrifuge so that when the centrifuge is operating the cavity is evacuated and is located radially outward of the dispensing opening of the dispensing vessel and centrifugal force will result in the fluid being drawn out of the dispensing opening and flowing into the cavity. 7. Apparatus as claimed in Claim 6, wherein the dispensing vessel comprises a cup with an open top and a narrow bore in its base to form the dispensing opening. 8. Apparatus as claimed in Claim 6 or Claim 7, wherein the centrifuge includes a plurality of dispensing vessels and a plurality of container support locations so that a plurality of cavities can be filled in one centrifuge operation. 9. Apparatus as claimed in any one of Claims 6 to 8, wherein the dispensing vessels and the container support locations are mounted together in swing-out buckets so that the open tops of the dispensing vessels are uppermost until the centrifuge starts to operate. 10. A method of introducing a fluid into a cavity in a container, substantially as herein described with reference to the accompanying drawings. 11. Apparatus for introducing a fluid into a cavity in a container, substantially as herein described with reference to the accompanying drawings.

Inventors: MCGRANE KEVIN, MCGRANE, KEVIN
Applicants: ANGLIA ELECTRONIC TECH LTD, MCGRANE KEVIN, ELECTRONIC TECHNIQUES (ANGLIA) LIMITED, MCGRANE, KEVIN
Application year: 1994
Main classification: B29C39
IPC codes: B29C31, B29C39, B29C70